Superlattice fabrication for InAs/GaSb/AISb semiconductor structures

ABSTRACT

A semiconductor structure and a method of forming same is disclosed. The method includes forming, on a substrate, an n-doped collector structure of InAs/AlSb materials; forming a base structure on said collector structure which base structure comprises p-doped GaSb; and forming, on said base structure, an n-doped emitter structure of InAs/AlSb materials. The collector and emitter structure are preferably superlattices each comprising a plurality of periods of InAs and AlSb sublayers. A heterojunction bipolar transistor manufactured using the method is disclosed.

FIELD OF THE INVENTION

The present invention pertains to an InAs/GaSb/AlSb semiconductorstructure useful in making bipolar junction transistors (BJTs), and moreparticularly useful in making heterojunction bipolar transistors (HBTs)and still more particularly useful for making npn HBTs having small(submicron) feature sizes. The present invention also pertains to amethod of making same.

BACKGROUND

HBT integrated circuits have found wide acceptance in industry for usein applications as diverse as satellite communication systems, radarsystems, cable television systems, optical receivers, etc. Prior artHBTs tend to be Gallium Arsenide (GaAs) devices. With the industrialacceptance of HBTs has come the need for HBT devices which can be madesmaller but without sacrificing the gain of the device and for HBTdevices which can operate at even higher frequencies than prior art GaAsdevices.

HBTs manufactured from an Indium Arsenide/Aluminum Antimonide/GalliumAntimonide (InAs/AlSb/GaSb) system possess a number of advantages overprior art GaAs HBTs. For example, GaSb is an excellent high-frequencyp-type base material, having higher hole mobility than presently usedbase materials, such as GaAs and In_(0.53)Ga_(0.47)As. It can be p-dopedwith Si to densities approaching 10²⁰/cm³, which are equal to thehighest densities achievable with GaAs and InGaAs (using C as a dopant).Since base resistance is inverse to the product of hole mobility anddoping level, the higher mobility translates to lower resistance forp-doped GaSb bases, which will increase the operating frequency limit ofa device.

An InAs/AlSb/GaSb material system is also preferable for HBTs beingfabricated with submicron feature sizes. Prior art HBTs have relativelylow surface Fermi level pinning energy, and as a consequence suffer fromrecombination of carriers at mesa sidewalls resulting in substantialsurface depletion effects. As feature dimensions shrink, these surfaceeffects become proportionally more significant, limiting the sizereduction which can be achieved without excessive loss of deviceperformance in terms of the gain of the device. By contrast, the surfacepinned energy for GaSb is near the valence band maximum. Accordingly, ap-type GaSb base layer would not have significant surface depletioneffects at mesa sidewalls, and could thus be scaled down with less lossof gain due to such surface effects.

In addition to the above advantages, the InAs/AlSb/GaSb material systemallows very flexible bandgap engineering. InAs, AlSb, and GaSb havenearly equal lattice constants, such that varying combinations of thematerials may be fabricated, in reasonable thicknesses, withoutsuffering serious crystalline defects. Consequently, flexibleengineering is possible which will permit implementation of advancedfeatures, such as drift fields in the base material to sweep minoritycarriers across the base to the collector.

A pnp InAs/AlSb/GaSb structure has been tested, as reported by Pekariket al., “An AlSb—InAs—AlSb double-heterojunction P-n-P bipolartransistor,” J. Vac. Sci. Technol. B, volume 10 no. 2, March/April 1992,pps. 1032-1034. This device does not employ either a GaSb base nor asuperlattice in the emitter or collector, and is not of the preferrednpn structure. Npn HBT devices are generally preferred by those skilledin the art for high performance applications.

Although desirable, InAs/AlSb/GaSb heterostructure systems have beendifficult to fabricate. First, the available emitter materials, AlGaSbor AlSb, require Tellurium (Te) for n-type doping. Tellurium isinconvenient for use in Molecular Beam Epitaxy (MBE) systems, becauseits memory effects make it difficult to avoid unwanted Te in subsequentlayers, and because the Te ties up an available port. Second, AlGaSb andAlSb have a conduction band mismatch with the preferred GaSb basematerials. If they are to be used as emitter materials, they needsophisticated grading to deal satisfactorily with the conduction bandoffset. They are even less desirable as collector materials, because thenoted conduction band offset can cause a trapping of carriers.

Accordingly, there is a need for an InAs/AlSb/GaSb HBT structure havinga GaSb base, and having improved conduction band alignment across thebase-emitter and base-collector junctions. Ideally, such an HBT would beeasy to dope. The present invention addresses these needs by employingan InAs/AlSb superlattice which can be constructed to achieve nearlyperfect conduction band alignment with GaSb. The entire HBT structurecan be doped using only Si for both n-doping of the emitter andcollector and for p-doping of the GaSb base. Moreover, the InAs/AlSbsuperlattice has a valence band offset to GaSb of approximately 0.475 V,enhancing the gain characteristics of devices fabricated according tothe present invention.

Si-doped InAs/AlSb superlattices have been used as n-type claddinglayers for infrared lasers, as described in U.S. Pat. No. 5,594,750 toHasenberg and Chow. They have also been used as Schottky barrier layers[Chow, Dunlap, et al., IEEE ElectronDevice Letters, Vol. 17, p. 69(1996)].

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of formingan InAs/GaSb/AlSb structure which can be used in the manufacture of HBTswhich permits conduction-band alignment of the junctions.

Preferably, the layers of the structure can be easily doped to desireddensities and therefore the use of Te as a dopant can be avoided.

Briefly, and in general terms, the present invention provides a methodof forming a semiconductor structure comprising the steps of: (i)forming, on a substrate, an n-doped collector structure of InAs/AlSbmaterials; forming a base structure on said collector structure whichbase structure comprises p-doped GaSb; and forming, on said basestructure, an n-doped emitter structure of InAs/AlSb materials.

Preferably the collector and/or emitter structures are provided bysuperlattice structures having sublayers of InAs and AlSb withthicknesses selected to yield a conduction band edge for thesuperlattice structures approximately equal to the conduction band edgeof GaSb.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the conduction and valence band edges of an InAs/AlSbsuperlattice as a function of constituent layer thickness;.

FIG. 2 is a diagram of the band energies for the preferred emitter, baseand collector; and

FIG. 3 depicts the structure of a semiconductor structure according tothe present invention;

FIGS. 3A-3E depict details of the structure shown by FIG. 3;

FIG. 4 depicts the structure of an alternative embodiment of thesemiconductor structure of FIG. 3 with a superlattice base; and

FIG. 5 depicts how the structures of FIGS. 3 and/or 4 may be etched andmetallized in order to provide an HBT device.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a graph of the conduction and valence band edges of anInAs/AlSb superlattice as a function of constituent layer thicknesses.In this case the InAs sublayer thickness is equal to the AlSb sublayerthickness in each constituent layer (or period) of the superlattice. Theconduction band edge of the superlattice can be varied over a widerange, including values above and below the GaSb conduction band (E_(c)^(GaSb)) at about 1.2 eV above the valence band maximum of InAs. As canbe seen, the conduction band of the superlattice (E_(c) ^(sls)) willequal the GaSb conduction band when the thickness of the constituentInAs and AlSb sublayers equal about 7.5 Å. This fact will prove usefulwhen selecting the thicknesses of certain InAs and AlSb layers insuperlattice structures used in the preferred embodiment of the presentinvention to be discussed with reference to FIG. 3. In contrast to theconduction band energy, the valence band energy of the superlattice(E_(v) ^(sls)) does not change significantly with layer thickness and islocated 400 meV below the valence band maximum of GaSb (E_(v) ^(GaSb)).

FIG. 2 is a flat band diagram for a HBT structure with a n-typeInAs/AlSb superlattice emitter, p-type GaSb base and an n-type InAs/AlSbsuperlattice collector. The diagram shows both the bulk InAs and AlSbband edges and the effective superlattice band edges in the emitter andcollector layers of the HBT device. Preferably, the constituentsuperlattice layer thicknesses are selected such that the conductionband edges in both the emitter and collector align with the conductionband edge in the GaSb base so that there is a zero conduction bandoffset, while at the same time the valence band edges are appropriatelymisaligned by approximately 400 meV. How this can be accomplished willbe described with reference to FIG. 3.

Superlattice structures are well known in the art. For additionalinformation the reader is directed to “Structural and transportproperties of InAs/AlSb superlattices” by D. H. Chow et al. published inthe Journal of Crystal Growth vol 150 (1995) at pages 879-882, thedisclosure of which is hereby incorporated herein by this reference. Thereader is also directed to “InAs/AlSb/GaSb Resonant Interband TunnelingDiodes and Au-on-InAs/AlSb-Superlattice Schottky Diodes for LogicCircuits” by D. H. Chow et al published in IEEE Electron DevicesLetters, Vol 17, No. 2, February 1996, the disclosure of which is herebyincorporated hereby by this reference.

FIG. 3 shows an epitaxial structure for making a HBT in accordance withthe present invention. Substrate 302 is preferably GaSb at the presenttime, although it is anticipated that continued improvements incompliant substrates may permit the use of other substrate materials,such as GaAs or InP. Subsequent layers are preferably grown by standardMolecular Beam Epitaxy (MBE) techniques, though any technique capable ofproviding the correct layer structure would be satisfactory. An undopedGaSb buffer layer 304 is grown on substrate 302 to a thickness of about2000 Å. A subcollector 306, grown upon buffer layer 304, is preferablyInAs, n-doped to a density of about 10¹⁹/cm³ using Si, and is grown to athickness of about 2000 Å. InAs provides nearly perfect ohmic contact tothe non-alloyed metallization, not shown, which will be deposited onsubcollector 306 to provide the collector connection. The preferredmetallization is Gold Germanium (AuGe), although Au and Al (Aluminum)are also considered to be satisfactory metals for the contacts formedfor the emitter, base and collector.

An optional subcollector grading layer 308 is preferably a chirpedsuperlattice which shifts the effective collector composition graduallyto lattice match the InAs preferred for subcollector 306, therebyreducing charge-impeding band discontinuities between collector 312 andsubcollector 306. Subcollector grading layer 308 includes about tengrading periods 310, each period 310 having a sublayer 307 of AlSb and asublayer 309 of InAs. Each period 310 may be conveniently of the samethickness, preferably about 50 Å, but the thicknesses of the periods 310may range from about 2 to 100 Å and the periods 310 need not necessarilyall be of the same thickness. Preferably, n-doping of subcollectorgrading layer 308 is effected to a desired doping level of about1*10¹⁹/cm³ by doping only InAs-containing sublayers 309 with Si, to adensity equal to the desired doping level divided by the proportion ofthe InAs-containing sublayer 309 within the particular period 310.Preferably, the thicknesses of the AlSb sublayers 307 increase with thethicknesses of the associated InAs sublayers 309 decreasing as theperiods 310 progress from the period immediately adjacent subcollector306 towards collector 312.

For an example, consider the layer structure of grading layer 308 in thepreferred embodiment, as shown in FIG. 3 and as shown in even greaterdetail by FIG. 3A. FIG. 3A shows a small portion of layer 306 and thefirst two periods immediately adjacent layer 306, namely the immediatelyadjacent period 301-1 and the next following period 310-2, as well assublayer 307 of the third period. In the first period 310-1 ofsubcollector grading layer 308, sublayer 307 comprises a layer of AlSbwith a thickness of preferably 1/22 of the period 310 thickness (whichis preferably 50 Å) so the thickness of sublayer 307 in the first period310-1 is preferably about 2.273 Å thick, while sublayer 309 comprises alayer of InAs having a thickness of 21/22 of the period 310 thickness,so the thickness of sublayer 308 in the first period 310-1 is preferablyabout 47.727 Å. The thickness of the first period 310-1 is 50 Å, since,as indicated above, 50 Å is the preferred thickness for each period 310.In each subsequent period 310, sublayer 307 increases in thickness byabout 1/22 of the period thickness (or about 2.273 Å) while sublayer 309decreases in thickness by about the same amount. Thus, the thickness ofAlSb sublayer 307 of the tenth period 310-10 is preferably 10/22 of theperiod 310 thickness (or about 22.73 Å), while the thickness of the InAssublayer 309 of the tenth period 310-10 is preferably 12/22 of theperiod 310 thickness (or about 27.27 Å). Since the desired averagedoping density is preferably 2*10¹⁸/cm³, the InAs sublayer 309 in thefirst period 310-1 preferably is doped to (50/47.727)×2*10¹⁸/cm³, orabout 2.1*10¹⁸/cm³, while the InAs sublayer 309 in the tenth periodpreferably is doped to (50/27.27)×2*10¹⁸/cm³, or about 3.67*10¹⁸/cm³.

This technique results in a constant average doping density through theoptional grading layer 308. However, since the grading layer 308 isitself optional, the grading layer 308, if used, may be of a more simpleconstruction. For example, the preferred average doping density of2*10¹⁸/cm³ for the grading layer 308 could be maintained by keeping thedopant concentration in each InAs sublayer 309 constant as opposed toadjusting the doping depending on the thickness of each sublayer 309 ofInAs.

Collector 312 is preferably provided by a superlattice of InAs and AlSb,grown to a thickness of preferably 3000 Å. Turning also to FIG. 3B, asmall portion (two periods 314-1 and 314-2 are identified) of thesuperlattice collector 312 immediately adjacent the tenth period 301-10of the grading layer 308 is depicted. Superlattice periods 314 each havea sublayer 311 of AlSb and a sublayer 313 of InAs. If, within eachperiod 314, the thickness of the AlSb sublayer 311 is equal to thethickness of its adjacent InAs sublayer 313, the resultant superlatticecollector 12 is very nearly lattice-matched to GaSb, having a mismatch(Δa/a) of only 5×10⁻⁴. As has been explained with reference to FIG. 1,the conduction band energy of the superlattice collector 312 is afunction of the thicknesses of sublayers 311 and 313. Sublayerthicknesses of about 7.5 Å for the sublayers 311 and 313 in the periods314 of the superlattice are preferred (particularly near the base 316)in order for the conduction band energy at the edge of the collectorsuperlattice 312 to align with the conduction band of the base 316 ofthe HBT, which base 316, as will be seen, is preferably formed of dopedGaSb. In order to achieve an effective average doping density of about10¹⁶/cm³, the AlSb sublayers 311 are preferably undoped while the InAssublayers 313 are preferably doped to about 2*10¹⁶/cm³. The overallthickness of the collector 312 is preferably 3000 Å. The thickness ofeach period 314 is preferably 15 Å, at least adjacent the base 316. Ifthe 15 Å period 314 thickness were maintained throughout the entirepreferred 3000 Å thickness of the collector 312, then the collector 312would comprise approximately 200 periods 314. Preferably, however, theperiods 314 increase slightly in thickness as the periods are more andmore remote from the base, so that the period 314 thickness preferablyincreases to about 20 Å in the center of the base 316.

In the embodiment of FIG. 3, base 316 is preferably bulk GaSb grown to athickness of preferably 300 Å. The GaSb base is p-doped preferably withSi to a density of about 10²⁰/cm³.

Emitter 322, grown upon base 316, is preferably a superlattice havingrepeating periods 324, each period 324 including a sublayer 323 of AlSband a sublayer 321 of InAs. FIG. 3C shows two periods 324-1 and 324-2,period 324-1 being the period 324 immediately adjacent base 316. It isgenerally preferred to align the conduction band energies of both thebase-emitter and base-collector junctions. In this preferred embodiment,since base 316 is bulk GaSb, emitter 322 is preferably grown in astructure similar to that of collector 312 in order to attain the sameconduction band energy of about 1.2 eV above the valence band maximum ofInAs. Accordingly, sublayers 323 and 321 are each preferably 7.5 Åthick. Emitter 322 differs from collector 312 in that it is preferablygrown to a lesser thickness (of about 1000 Å) and is doped more heavilythan the collector 312. Emitter 312 is preferably n-doped, using Si, toa density of about 10¹⁸/cm³, by doping the InAs sublayers 321 to adensity of 2*10¹⁸/cm³. In that way the AlSb sublayers 323 may be leftundoped.

Emitter contact grading layer 318 is somewhat similar to thesubcollector grading layer 308 in this embodiment, the emitter contactgrading layer 318 including preferably a number of periods 320, startingwith a period thickness of about 15 Å at the juncture with the emitterlayer 322 and increasing to a period thickness of about 50 Å thickadjacent the InAs contact layer 330. Each period 320 comprises an InAssublayer 319 and a AlSb sublayer 317. FIG. 3D is a detailed view of afirst few periods 320-1 through 320-3 of the emitter contact gradinglayer 318 immediately adjacent the last two periods 324-X and 324-X-1 ofthe emitter 322. If the total thickness of the emitter is indeed about1000 Å, then X (the number of periods 324 in the superlattice emitter322) will fall in the range of about 65 to 70. The proportionalthickness of each InAs sublayer 319 within its period 320 convenientlyincreases each period from about 11/22 in the first period 320-1 nearestemitter 322 to about 21/22 in the last period 320-X immediately adjacentemitter contact 330. FIG. 3E is a detailed view of the last two periods310-X-1 and 310-X next to the emitter contact 330. The thickness of eachof the periods 320 is conveniently increased to about 50 Å whiledecreasing the proportional thickness of each AlSb sublayer 317 fromabout 11/22 of the first period 320-1 to about 1/22 of the last period320-X. Emitter contact grading layer 318 has an overall preferredthickness of about 1000 Å. Doping is accomplished as in subcollectorgrading layer 308, by using Si n-type doping of the InAs sublayers 319to achieve an average doping density of preferably 2*10¹⁸/cm³ in emittercontact grading layer 318.

In the preferred embodiment of FIG. 3, emitter contact layer 330 ispreferably bulk InAs grown to a thickness of preferably 300 Å. The InAsemitter contact 330 is n-doped preferably with Si to a density of about10¹⁹/cm³.

In the embodiment of FIG. 3, the base layer is described as beingprovided by bulk GaSb grown using known epitaxial techniques, such asMBE. However, instead of using a bulk material for the base, if a gradedAlGa Sb alloy is provided instead, then a drift field for electronscrossing the base layer 316 can be provided and the emitter superlatticeenergy gap can be widened slightly to provide conduction band edgealignment at both the emitter-base and collector-base junctions. Forexample, Al_(0.2)Ga_(0.8)Sb has a higher conduction band maximum thandoes GASb by about 100 meV. As such the InAs/AlSb superlattice emitterperiod 324 would have a smaller thickness in order to provide theappropriate alignment with the E_(C) for Al_(0.2)Ga_(0.8)Sb.

FIG. 4 shows another alternative embodiment of the base 316. Here thebase 316 is a chirped superlattice, graded in ten periods 317 from GaSbat the collector junction to Al_(x)Ga_(1−x)Sb at the emitter junction.The Al_(x)Ga_(1−x)Sb composition may have x at the emitter end of from 0to about 0.2. However, x is preferably equal to 0.1 and thus the basematerial 317 adjacent the emitter is preferably formed byAl_(0.1)Ga_(0.9)Sb.

The epitaxially grown structures shown in FIG. 3 or 4, after suitablemasking and etching using well known techniques, can be etched to obtainthe shape of the structure shown in FIG. 5. With the application ofsuitable metalization, the HBT structure shown in FIG. 5 is obtained.The metalization includes forming collector contacts 340, base contacts345 and emitter contacts 350. Since masking and etching using thestructures shown in FIG. 3 or 4 to attain the structure shown in FIG. 5is rather straightforward for those skilled in the art, the details forhow the masking and etching is carried out is a matter of design choice.

Having described the invention in connection with its preferredembodiments, modification will now suggest itself to those skilled inthe art. As such the invention is not to be limited to the disclosedembodiments, expect as required by the appended claims.

What is claimed is:
 1. A semiconductor structure comprising: asubstrate, a n-doped collector structure of a superlattice of InAs/AlSbmaterials disposed on said substrate; a base structure on said collectorstructure, said base structure including p-doped GaSb; and a n-dopedemitter structure on said base structure, said emitter structureincluding a superlattice of InAs/AlSb materials.
 2. The semiconductorstructure according to claim 1 wherein said emitter structure includesan emitter superlattice having sublayers containing InAs and sublayerscontaining AlSb.
 3. The semiconductor structure according to claim 2wherein said emitter structure further includes an emitter contact layerof InAs.
 4. The semiconductor structure according to claim 3 wherein thesaid emitter superlattice further includes a chirped superlattice layerbetween the emitter superlattice and the emitter contact layer, thechirped superlattice at least partially grading a transition between theemitter superlattice having sublayers containing InAs and sublayerscontaining AlSb and said emitter contact layer of InAs.
 5. Thesemiconductor structure of claim 4 wherein the thicknesses of thesublayers containing InAs and AlSb vary to shift an effectivecomposition of the emitter superlattice gradually to match InAs of theemitter contact layer.
 6. The semiconductor structure according to claim2 wherein the emitter superlattice comprises a number of periods ofsublayers containing InAs and sublayers containing AlSb.
 7. Thesemiconductor structure according to claim 6, wherein each sublayer ofat least one period has a thickness of approximately 7.5 Å.
 8. Thesemiconductor structure according to claim 6 wherein the periods of theemitter superlattice are thicker in the center of the emittersuperlattice than they are adjacent the base structure.
 9. Thesemiconductor structure according to claim 2 wherein each sublayer has athickness selected to yield a conduction band edge approximately equalto a conduction band edge of GaSb.
 10. The semiconductor structureaccording to claim 1 wherein said collector structure includes acollector superlattice having sublayers containing InAs and sublayerscontaining AlSb.
 11. The semiconductor structure according to claim 10wherein said collector structure further includes a subcollector layerof InAs.
 12. The semiconductor structure according to claim 11 whereinsaid collector structure further includes a chirped superlattice layerdisposed between the collector superlattice and the subcollector layer,the chirped superlattice at least partially grading a transition betweenthe collector superlattice having sublayers containing InAs andsublayers containing AlSb and said subcollector layer.
 13. Thesemiconductor structure of claim 12 wherein the step of forming thechirped superlattice includes controlling the thicknesses of thesublayers containing InAs and AlSb to shift the effective collectorcomposition gradually to match the InAs of the subcollector layer. 14.The semiconductor structure according to claim 10 wherein the emittersuperlattice comprises a number of periods of sublayers containing InAsand sublayers containing AlSb.
 15. The semiconductor structure accordingto claim 14, wherein each sublayer of at least one period has athickness of approximately 7.5 Å.
 16. The semiconductor structureaccording to claim 14 wherein the periods of the emitter superlatticeare thicker in the center of the emitter superlattice than they areadjacent the base structure.
 17. The semiconductor structure accordingto claim 10 wherein each sublayer has a thickness selected to yield aconduction band edge approximately equal to the conduction band edge ofGaSb.
 18. The semiconductor structure according to claim 1 wherein theemitter structure is more heavily doped than is the collector structure.